The present invention relates to multicylinder petrol engines provided with a system for variable actuation of the valves.
The applicant has already proposed, in its U.S. Pat. No. 6,237,551, an internal-combustion engine of the type comprising:
at least one intake valve and at least one exhaust valve for each cylinder, each valve being provided with respective elastic return means, which push the valve towards a closed position for controlling respective intake and exhaust pipes; and
at least one camshaft, for actuating the intake and exhaust valves of the engine cylinders by means of respective tappets;
in which each intake valve is controlled by the respective tappet against the action of the aforesaid elastic return means by interposition of hydraulic means that include a pressurized fluid chamber;
said pressurized fluid chamber being designed to be connected by means of a solenoid valve to an exhaust channel in order to uncouple the valve from the respective tappet and bring about fast closing of the valve as a result of the respective elastic return means;
electronic control means for controlling each solenoid valve so as to vary the time and the opening stroke of the respective intake valve according to one or more operating parameters of the engine.
The purpose of the present invention is to further improve the previously proposed engine in order to provide a petrol engine that affords a series of advantages as regards control of combustion, reduction of consumption, reduction of harmful exhaust gases, and, in general, improvement of efficiency of the engine.
With a view to achieving the above purpose, the subject of the present invention is a petrol engine of the known type referred to above, characterized in that:
the engine is provided with second electronic control means for keeping the exhaust valves closed;
the aforesaid electronic control means are pre-arranged for keeping the intake valves of one or more selected cylinders of the engine closed during the normal operating cycle, simultaneously with the activation of the aforesaid second means, which keep the exhaust valves of said selected cylinders closed, so as to inhibit operation of said selected cylinders, thus obtaining an engine with displacement that can be varied with modularity.
It should be considered that the concept of modularity, taken in itself, in petrol engines is known. The de-activation of one or more cylinders of the engine is performed, in general, to achieve a reduction in specific consumption, thanks to a decrease in the work of pumping of the cylinders that are not in the combustion phase, and a decrease in the organic losses, combined with an improvement in combustion efficiency. In this way, a variation in engine displacement, which is controlled according to the torque requirement of the vehicle, is obtained.
The system according to the invention enables adjustment of engine load without the use of a throttle valve and, in itself, thus tends to minimize pumping losses. In addition to the above advantage, the system according to the invention, which envisages simultaneous closing both of the intake valves, via a hydraulic system of variable control of the valves, and of the exhaust valves, enables the aforesaid result to be achieved with higher efficiency using relatively simple and inexpensive means. The aforesaid means, which are designed to keep the exhaust valves closed, may be obtained in any known way, for example, by means of tappets of the so-called xe2x80x9ccollapsiblexe2x80x9d type, produced by the German firm INA.
In the case where the engine has an even number of cylinders, modularity is obtained, according to the invention, by de-activating one half of the cylinders of the engine. In this way, a number of disadvantages are achieved: given the same mean effective pressure (MEP) of the engine, an MEP of the active cylinder is found to be twice as high. This determines an improvement in the combustion efficiency of the cylinders in firing, with a consequent reduction in specific consumption. By de-activating one half of the cylinders, also a reduction in losses due to friction is obtained. This result is justified by the fact that, in the modular configuration, there is no longer any absorption caused by the cycles of compression and release of the springs of the intake valves and by the fact that the loads acting on the crank mechanism, on the pistons, and on the corresponding segments are considerably reduced. Owing to the fact that, in the modular configuration, also the exhaust valves are closed, there is further obtained a greater reduction in losses due to friction, in so far as, added to the reduction in losses due to friction on account of closing of the intake valves, there is the reduction in losses corresponding to the de-activation of the exhaust valves, whilst, above all, the increase in losses due to pumping, which is proper to the modularity obtained with just the inhibition of the intake valves, does not occur.
In the case where the number of cylinders of the engine is odd, the system envisages an inhibition of operation, by turns, of all the cylinders of the engine. In fact, if the aim is to maintain uniformity of staggering between the individual combustion phases, in order to obtain a balanced engine, it is not possible to resort to the permanent de-activation of just certain predetermined cylinders. If it is considered that five cylinders are present having the order of firing 1, 2, 4, 5, 3, this, in the case of fixed de-activation of the cylinders, could become 1, (2), 4, (5), 3, where the numbers in brackets indicate the inactive cylinders. It is evident that, whilst between the combustions 1-4 and 4-3 there are 288xc2x0 of engine angle, between the ones corresponding to the sequence 3-1 there are only 144xc2x0. The system according to the invention makes it possible to overcome this type of problem easily, by providing an eight-stroke cycle. In practice, once again with reference to the example of five cylinders, the order of firing 1, 2, 4, 5, 3 becomes 1, (2), 4, (5), 3, (1), 2, (4), 5, (3), where, as above, the numbers in brackets indicate the inactive cylinders. It may be seen that in this case the periodicity of the cycle is twice that already specified above in the case of engines with an even number of cylinders.
According to a further peculiar characteristic of the invention, the aforesaid electronic control means with which the engine is provided are pre-arranged for performing an adjustment of the effective compression ratio (ECR) in order to obtain a control of combustion.
The phenomenon of combustion is controlled by the values of pressure and temperature that are reached at the end of the compression phase. Complete flexibility in management of the angle of closing of the intake valves, which is proper to the system of variable actuation of the valves with which the engine according to the invention is provided, enables a combustion cycle to be obtained in which the effective compression ratio (ECR) is independent of the geometrical compression ratio (GCR). A system that enables variation of the end-of-compression temperature, given the same pressure, through management of the effective compression ratio (ECR) is a system that can control combustion.
The engine according to the invention achieves the above purpose in two different possible modes:
1) by anticipating closing of the intake valve prior to bottom dead centre (early intake-valve closing); and
2) by retarding closing of the intake valve to exceed the angle of maximum volumetric efficiency of the engine (late intake-valve closing).
The system according to the invention affords the possibility of having a variable effective compression ratio and one that is used for every point of operation of the engine, and, at the same time, a geometrical compression ratio that is higher than the one normally to be found in petrol engines that adopt conventional distribution systems. In fact, the two compression ratios are independent of one another in that closing of the intake valves is not mechanically phased to the crank mechanism of the engine. The above is an advantage in so far as, if, on the one hand, the high partial loads ECR and GCR improve engine emission (faster and more stable combustion due to higher temperatures in the combustion chamber, and a smaller amount of unburnt fuel in the combustion chamber) and consumption (or efficiency of the cycle), on the other hand, thanks to the hydraulic system of variable control of the valves, it is possible, at full load, to set an ECR value such that the performance of the engine is not limited by the phenomenon of spark knock. This may occur, as mentioned previously, according to the two different modes described above. According to the first mode, associated to the early-intake-valve-closing strategy, to the reduction in the effective compression ratio, and to the consequent reduction in the end-of-compression temperature deriving therefrom, is an increase in the density of the fresh charge introduced through the use of a compressor. The charge drawn in will, at the end of the compression phase, be at a pressure similar to that of a conventional cycle, but since its temperature is lower (on account of the smaller effective compression ratio), it will enable a greater quantity of drawn charge to be entrapped and will be unfavourable to the onset of spark knock, with obvious benefits in terms of engine performance.
According to the second mode mentioned previously, the reduction in the effective compression ratio is obtained by retarding closing of the intake valve. The effect produced is similar to that of the first mode mentioned above.
According to a further preferred characteristic of the invention, the camshaft comprises cams with a profile such as to enable opening of the respective intake valves controlled by them not only during the conventional opening phase within the normal operating cycle of the engine, but also in some additional phases of the cycle. The aforesaid electronic control means are designed to cause opening of each solenoid valve so as to keep the respective intake valve closed during the above-mentioned conventional phase and/or during one or more of said additional phases in which the respective cam would tend to cause opening of the valve, so that the engine may be made to function selectively according to different operating modules by controlling said solenoid valves.
Preferably, the electronic control means are designed to enable a second opening of the intake valve during the exhaust phase of a given cylinder so as to obtain intake of the hot gases that have flowed into the intake pipe during the subsequent induction phase, with consequent exhaust-gas recirculation (EGR).
The flexibility in management of the valves guaranteed by the hydraulic system of variable actuation of the valves makes it possible to manage exhaust-gas recirculation (EGR) at intake by means of the second opening of the intake valve during the exhaust phase. In this way, an internal EGR is obtained in so far as the engine exhaust gases, which have flowed into the intake pipe, are re-introduced into the combustion chamber in the course of the subsequent induction phase. The re-opening of the intake valve during the exhaust phase is obtained by profiling the cam with a second appropriately phased cam. Thanks to the hydraulic system of variable actuation of the valves, it is possible to activate/de-activate said second opening of the intake valve according to the different engine requirements. In practice, the second opening of the intake valve will be active at partial loads, in order to reduce harmful exhaust emissions, whilst outside the cycle area and in the power curve it will be de-activated in order not to penalize combustion and engine performance. The maximum amount of gas recirculated will be obtained by controlling the solenoid valves of the hydraulic system of variable actuation of the valves in order to enable the intake valve to follow completely the profile of the second cam (full lift), whilst, by appropriately de-energizing the solenoid valve, it will be possible to reduce/vary said amount as desired, thus obtaining an extremely fine adjustment of the internal EGR.
Thanks to the use of the aforesaid hydraulic system of variable actuation of the valves, the engine according to the invention is moreover able to implement different strategies that enable the desired effects to be obtained in terms of reduction in consumption and in harmful exhaust emissions, etc., in a petrol engine.